Recently, with the recent development of digital information communication and electronics industries, research on memory devices such as DRAMs or flash memories based on charge control is expected to reach its limitations. To overcome the limitations, research on new memory devices using a phase change, a change in magnetic field, etc. has progressed. The new memory devices under research store information using the principle that the resistance of a material is changed by a change in state of the material.
In the case of a flash memory as a representative of nonvolatile memory devices, a high operating voltage is required to write or erase data. Thus, when the flash memory is scaled down below 65 nm, there may be some limitations due to interference between adjacent cells, and the slow operating speed and high power consumption are still problematic.
Among various nonvolatile memory devices which have recently been developed, a ferroelectric RAM (FeRAM) has a problem of material instability, and a magnetic RAM (MRAM) has problems of complex manufacturing process, complex multilayer structure, and low margin of write/read operations. Thus, the development of next generation nonvolatile memory technology capable of replacing these memory devices will be a key area of research.
A resistive random access memory (hereinafter referred to as “ReRAM”) implements the operation of the memory using the phenomenon that the resistance of a thin film is changed by a voltage applied to the thin film. The ReRAM has outstanding advantages of no deterioration due to continuous recording and reproducing of data, operability at high temperature, nonvolatile properties, and stability of data. Moreover, when an input pulse is applied to the ReRAM, it can operate at a high speed of about 10 to 20 ns with a resistance change of about 1,000 times.
Most of the ReRAM devices have a resistance-variable layer with a single film structure, and thus the ReRAM devices have advantages of high integration and high operation speed. Moreover, the existing CMOS process and integration process technologies can be applied to the resistance-variable layer. An oxide is used as a material for the resistance-variable layer and, in particular, a binary oxide or perovskite oxide is used. Recently, a perovskite oxide doped with metal is used.
Korean Patent Publication No. 10-2006-0083368 discloses a ReRAM with a multilayer film comprising metal oxides having different composition ratios, in which NiOx, HfOx, TiOx, Ta2Ox, Al2Ox, La2Ox, Nb2Ox, SrTiOx, Cr-doped SrTiOx, or Cr-doped SrZrOx (where x is 1.5 to 1.9) is used as the metal oxides.
Korean Patent Publication No. 10-2006-0106035 discloses a ReRAM device comprising a perovskite oxide of Cr-doped SrZr3 as a resistive layer.
Moreover, Korean Patent Publication No. 10-2004-0063600 mentions a ReRAM device, in which a barrier layer of Ta, TaN, Ti, TiN, TaAlN, TiSiN, TaSiN, TiAl or TiAl is formed on an Ir substrate and a thin film of Pr0.7Ca0.3MnO3 (hereinafter referred to as “PCMO”) as a resistive layer is formed on the barrier layer. The ReRAM device is prepared by repeating coating, baking and annealing processes until a PCMO layer has a desired thickness, and thus the entire process is very complicated. Moreover, the main processes are performed under atmospheric conditions, which may affect the characteristics of ReRAM due to oxidation and surface contamination and may impose limitations on the stabilization of thin films. Moreover, it is difficult to ensure excellent reproducibility due to the instability of operating voltage and resistance caused by difficulties in controlling the stability of the point defect structure in oxide thin films for ReRAMs produced by the above-mentioned inventions, and it is further difficult to provide stabilization of device operation due to process limitations.
Therefore, it is necessary to develop a nonvolatile memory device, which can be produced by a simple process, can have various resistances by controlling the operating voltage, has no possibility of surface contamination, and thus can be applied in various application fields.